The present invention generally pertains to a process for producing a gas-tight, sealed alkaline battery having a negative electrode whose excess capacity is subdivided into charge reserve and discharge reserve by pretreatment of the electrode outside of the cell.
In a battery operating under gas-tight conditions, the capacities of the positive and negative electrodes must be designed in such a way that both the required rated capacity of the cell, and operation of the cell during overload and high-current load, is guaranteed. To meet this requirement, suitable capacities are developed by selecting electrodes of different sizes, in such a way that they are, for example, in the following ratio to each other: EQU (+):(-)=1:1.5
This ratio is dependent, on an individual case basis, upon the special requirements which are to be imposed on the cell, for example, protection from overcharging. In addition, the drop in capacity of the negative electrodes, which is inevitable during operation, must be compensated.
A number of such processes are known. According to the majority of these processes the negative electrode is precharged in comparison with the positive electrode. The extent of this precharging corresponds to the desired discharge reserve. As an example, chemical reducing agents are suitable for this precharging. Thus, according to DE-OS No. 28 51 463, an electrolyte containing methanol, ethanol, or propanol as additives may be introduced into a cell equipped with partially discharged electrodes. According to U.S. Pat. Nos. 3,288,643 and 3,297,433, reducing agents such as aluminum powder or zinc powder may be mixed into a non-charged negative active mass to exert their specific action on the negative electrode, i.e. the reduction of a certain amount of Cd(OH).sub.2 into Cd.sub.met. This provides a result which is similar to the above-mentioned alcohols, only in a sealed cell filled with electrolyte.
Other known process develop the negative excess capacity indirectly, via a special pretreatment of the positive electrode. Thus, according to DE-OS No. 30 26 073, part of the uncharged positive electrode mass is electrochemically reduced in hot KOH to metallic Ni before assembly with a completely discharged negative electrode. During charging of the resulting sealed cell, the electrochemical reduction is undone, such that charged cadmium in a quantity equivalent to the oxidized Ni is formed in the negative electrode. Since the Ni.fwdarw.Ni.sup.2+ transition linked with the formation of this Cd.sub.met is not reversed during subsequent operation of the cell, i.e. it is irreversible, the portion of Cd developed in the negative electrode is obtained as a discharge reserve after each exhaustive discharge of the cell. According to DE-PS No. 21 56 554, it is likewise possible to generate a discharge reserve, in an irreversible manner, by mixing the antipolar mass of an uncharged positive electrode with metallic zinc before installation, which zinc reduces part of the antipolar zinc mass after installation and after addition of the electrolyte, and by forming a discharge reserve in the negative electrode during subsequent charging, which discharge reserve corresponds to the reduced portion of the antipolar mass.
There are also processes, such as that according to FR-PS No. 21 15 704, in which the cell balance is set to correspond to the desired design ratio for the electrodes by charging and overcharging of the cell, while open and cold. Alternatively, it is possible to generate a discharge reserve by partial reduction of Cd(OH).sub.2 electrodes in a strongly reducing chemical bath (DE-OS No. 29 43 101), or by thermal decomposition of cadmium formate impregnating negative sintered electrodes (DE-OS No. 25 07 988).
However, the foregoing processes exhibit certain disadvantages in practice. In some, the desired capacity ratio is balanced only after start-up charging. The use of chemical substances to oxidize or to reduce the electrodes can create a problem in that the reaction products cannot be easily removed from the cell, adversely influencing the characteristics of the cell under electric load. The alkali mists discharged with the charging gases during the electrochemical setting of the cell balance are extremely troublesome. In addition, such processes have the disadvantage that the production process cannot be sufficiently automated because of considerable differences in the duration of the processing steps.